The AIDS epidemic was discovered June 5, 1981, when the U.S. Centers for Disease Control and Prevention reported a cluster of Pneumocystis carinii pneumonia (now classified as Pneumocystis jiroveci pneumonia) in five homosexual men in Los Angeles.[91] Originally dubbed GRID, or Gay-Related Immune Deficiency, health authorities soon realized that nearly half of the people identified with the syndrome were not homosexual men. In 1982, the CDC introduced the term AIDS to describe the newly recognized syndrome.

Three of the earliest known instances of HIV infection are as follows:

A plasma sample taken in 1959 from an adult male living in what is now the Democratic Republic of Congo.[92]
HIV found in tissue samples from an American teenager who died in St. Louis in 1969.
HIV found in tissue samples from a Norwegian sailor who died around 1976.
Two species of HIV infect humans: HIV-1 and HIV-2. HIV-1 is more virulent and more easily transmitted. HIV-1 is the source of the majority of HIV infections throughout the world, while HIV-2 is less easily transmitted and is largely confined to West Africa.[93] Both HIV-1 and HIV-2 are of primate origin. The origin of HIV-1 is the Central Common Chimpanzee (Pan troglodytes troglodytes) found in southern Cameroon.[94] It is established that HIV-2 originated from the Sooty Mangabey (Cercocebus atys), an Old World monkey of Guinea Bissau, Gabon, and Cameroon.

Although a variety of theories exist explaining the transfer of HIV to humans, there is no widely accepted scientific consensus of any single hypothesis and the topic remains controversial. Freelance journalist Tom Curtis discussed one currently controversial possibility for the origin of HIV/AIDS in a 1992 Rolling Stone magazine article. He put forward what is now known as the OPV AIDS hypothesis, which suggests that AIDS was inadvertently caused in the late 1950s in the Belgian Congo by Hilary Koprowski's research into a polio vaccine.[95] Although subsequently retracted due to libel issues surrounding its claims, the Rolling Stone article motivated another freelance journalist, Edward Hooper, to probe more deeply into this subject. Hooper's research resulted in his publishing a 1999 book, The River, in which he alleged that an experimental oral polio vaccine prepared using chimpanzee kidney tissue was the route through which simian immunodeficiency virus (SIV) crossed into humans to become HIV, thus starting the human AIDS pandemic

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